1. Field of the Invention
This invention relates to an ultrasonic flowmeter for a fluid in a tube and, more particularly for liquids, affording a high level of accuracy over a wide flow-rate and also suitable for measuring small flow-rates.
2. Description of the Prior Art
Ultrasonic flowmeters are already known, particularly through French Pat. No. 2,281,571 (74-27312) filed Aug. 6, 1974, wherein two electroacoustic transducers in a tube are opposed along an axis oblique with respect to the axis of the tube in which the fluid whose speed or flow-rate is to be measured moves. Upon transmission, a pulse train of recurrence frequency f.sub.e, of around 5 MHz for example, and of period t.sub.e is sent to the two transducers simultaneously. During this time, the electronic receiver circuits are blocked. At the end of the transmission, the electronic receiver circuits related to the two transducers are switched to reception. These circuits, including amplifiers, comparators and frequency dividers, restore the shape of the signals received and divide the frequency of these pulses by a factor d so as to deliver a train of p pulses of recurrence frequency f.sub.e /d=1/t.sub.r.
The two pulse trains received, R.sub.1 and R.sub.2, after division of their frequency by d are characterized by the delay T.sub.1 of reception train R.sub.1 with respect to the transmission train and the delay T.sub.2 of reception train R.sub.2 with respect to the transmission train. The difference T=.+-.(T.sub.1 -T.sub.2) i.e. that between received pulse trains R.sub.1 and R.sub.2 is measured. This delay is the same for each of the pulses of the two trains taken two by two. It is thus possible to make up a signal R formed of p pulses of width T. The advantage of having p pulses lies in the fact that the uncertainties on the rise and fall times of these pulses can be reduced by summation in so far as these uncertainties are not correlated with the signal. The speed V of the flow is then given by: EQU V=(KT/T.sub.1 T.sub.2) (1)
where K is a coefficient dependent on the dimensions of the tube and the nature of the fluid.
Pulse trains R.sub.1 and R.sub.2 are produced by electronics described in the aforementioned patent. Delays T, T.sub.1 and T.sub.2 appearing in equation (1) are measured using constant-current analog integrators providing voltages proportional to these delays and the speed is calculated analogically from voltages proportional to T, T.sub.1 and T.sub.2 respectively.
The drawback of the flowmeter of the prior art is that its components have limited accuracy and are prone to drifting chiefly caused by the temperature. More specifically, the logic circuits used in the two reception channels can never be strictly identical. It is therefore desirable to take into account the fact that these two channels do not introduce equal delays in signals received but present a differential delay.